14 INTRODUCTION. 



All the animal fluids being drawn from the blood by secretion, we 

 can have no doubt that such is the case with the nervous fluid, and 

 that the medullary matter secretes it. 



On the other hand, it is certain that the medullary matter is the 

 sole conductor of the nervous fluid ; all the other organic elements 

 restrain and arrest it, as glass arrests electricity. 



The external causes which are capable of producing sensations or 

 causing contractions of the fibre are all chemical agents, capable of 

 effecting decompositions, such as light, caloric, the salts, odorous 

 vapours, percussion, compression, &c. &c. 



It would appear, then, that these causes act on the nervous fluid 

 chemically, and by changing its composition ; this appears the more 

 likely, as their action becomes weakened by continuance, as if the 

 nervous fluid needed the resumption of its primitive composition to fit 

 it for a fresh alteration. 



The external organs of the senses may be compared to sieves, which 

 allow nothing to pass through to the nerve, except that species of agent 

 which should affect it in that particular place, but which often accu- 

 mulates it so as to increase its effect. Thus, the tongue has its 

 spongy papillae which imbibe saline solutions; the ear, a gelatinous pulp 

 which is violently agitated by sonorous vibrations ; the eye, transparent 

 lenses which concentrate the rays of light, &c. &c. 



It is probable that what are styled irritants, or the agents which 

 occasion the contractions of the fibre, exert this action by producing on 

 the fibre, by the nerve, a similar effect to that produced on it by 

 the will ; that is, by altering the nervous fluid, in the way that is 

 requisite to change the dimensions of the fibre which it influences: 

 but with this process the will has nothing to do, and very often the 

 individual is unconscious of it. The muscles, separated from the body, 

 preserve their susceptibility of irritation as long as the portion of the 

 nerve that remains with them preserves the power of acting on them 

 with this phenomenon the will has evidently no connexion. 



The nervous fluid is altered by muscular irritation, as well as by 

 sensibility and voluntary motion, and the same necessity exists for the 

 re-establishment of its primitive composition. 



The transmutations necessary to vegetable life are occasioned by 

 irritants; the aliment irritates the intestine, the blood irritates the 

 heart, &c. These movements are all independent of the will, and 

 generally (while in health) take place without the knowledge of the 

 individual ; in several parts, the nerves that produce them are even 



